The present invention relates to a PTC device and more particularly to an electrode composition.
Conventional PTC devices comprise a PTC element, formed of a PTC composition, which is interposed between two electrodes. An electrode is attached to each electrode. An enclosure totally covers the device. The electrodes are made of metal sheet or metal foil, and each electrode is affixed, using a heat press, to opposing surfaces of the PTC element. When a metal foil is used for the electrode, the surface of the electrode contacting the PTC element is generally smooth. In some cases, the surface is roughened surface as disclosed in Japanese Patent Laid-Open No. 196901/1985 and No. 98601/1987.
Another conventional PTC device comprises a PTC element having two electrodes embedded therein. Each electrode is a combination of the electrode itself and a lead terminal.
The temperature of a conventional PTC device is approximately room temperature in the absence of voltage impressed across the electrodes. However, if a voltage is impressed and increased up to and over a trip voltage, which is the voltage at which the PTC behavior the PTC element is exhibited, the resistance of the PTC device increases. The current flow through the PTC device thus decreases and the power dissipation in the PTC device reaches an equilibrium so the temperature of the device stays at a temperature at or around the trip temperature of the device. Accordingly, the temperature of the PTC device varies from room temperature to the trip temperature of the PTC device with each application of a trip voltage. Therefore, the PTC element and electrodes repeatedly expand and contract during temperature cycles resulting from ON/OFF voltage application cycles.
In general, the coefficient of thermal expansion of a PTC element at a temperature below the trip temperature of the PTC element is larger than that of the electrode. Therefore, when the PTC element expands with voltage applied across the electrodes, the expansion of the electrodes does not track the expansion of the PTC element. This results in the electrodes peeling off the PTC element. The degree of the peeling depends on the difference between the coefficient of thermal expansion of the PTC element and that of the electrode.
In order to overcome this problem of electrodes peeling off a PTC element, Japanese Patent Laid Open No. 196901/1985 and No. 98601/1987 disclose an electrode having a rough surface affixed to a PTC element. Even with roughened contacting surfaces, if the difference between the coefficient of thermal expansion of the PTC element and that of the electrode, at a temperature under the trip temperature of the PTC element, is sufficiently large, peeling of the electrode off the PTC element may still occur.
Another problem is that, as a the peeling process proceeds, the area of ohmic contact between the electrode and the PTC element decreases. Consequently, the electrode-to-electrode resistance of the PTC device tends to increase in proportion to the degree of the peeling.